locking: Factorize LockSet.acquire

By moving the main code of LockSet.acquire to its own function
we reduce the code complexity a bit and clarify the exception
handling.

This also fixes a case where a lock acquire timeout wasn't
handled correctly, leading to obscure error messages.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>